RE: [PATCH v5 2/2] xen/x86: Livepatch: support patching CET-enhanced functions

[Jiamei Xie <Jiamei.Xie@xxxxxxx>]

Hi  Bjoern,

> -----Original Message-----
> From: Xen-devel <xen-devel-bounces@xxxxxxxxxxxxxxxxxxxx> On Behalf Of
> Ross Lagerwall
> Sent: 2022年3月10日 1:12
> To: Bjoern Doebel <doebel@xxxxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx
> Cc: Michael Kurth <mku@xxxxxxxxx>; Martin Pohlack
> <mpohlack@xxxxxxxxx>; Roger Pau Monne <roger.pau@xxxxxxxxxx>;
> Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>; Konrad Rzeszutek Wilk
> <konrad.wilk@xxxxxxxxxx>
> Subject: Re: [PATCH v5 2/2] xen/x86: Livepatch: support patching CET-
> enhanced functions
> 
> > From: Bjoern Doebel <doebel@xxxxxxxxx>
> > Sent: Wednesday, March 9, 2022 2:53 PM
> > To: xen-devel@xxxxxxxxxxxxxxxxxxxx <xen-devel@xxxxxxxxxxxxxxxxxxxx>
> > Cc: Michael Kurth <mku@xxxxxxxxx>; Martin Pohlack
> <mpohlack@xxxxxxxxx>; Roger Pau Monne <roger.pau@xxxxxxxxxx>;
> Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>; Bjoern Doebel
> <doebel@xxxxxxxxx>; Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>;
> Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
> > Subject: [PATCH v5 2/2] xen/x86: Livepatch: support patching CET-
> enhanced functions
> >
> > Xen enabled CET for supporting architectures. The control flow aspect of
> > CET expects functions that can be called indirectly (i.e., via function
> > pointers) to start with an ENDBR64 instruction. Otherwise a control flow
> > exception is raised.
> >
> > This expectation breaks livepatching flows because we patch functions by
> > overwriting their first 5 bytes with a JMP + <offset>, thus breaking the
> > ENDBR64. We fix this by checking the start of a patched function for
> > being ENDBR64. In the positive case we move the livepatch JMP to start
> > behind the ENDBR64 instruction.
> >
> > To avoid having to guess the ENDBR64 offset again on patch reversal
> > (which might race with other mechanisms adding/removing ENDBR
> > dynamically), use the livepatch metadata to store the computed offset
> > along with the saved bytes of the overwritten function.
> >
> > Signed-off-by: Bjoern Doebel <doebel@xxxxxxxxx>
> > Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> > CC: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
> 
> Reviewed-by: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>

Tested-by: Jiamei xie <jiamei.xie@xxxxxxx>

Cheers, 
Jiamei